Physico-chemical
Properties of 1‑Alkyl-3-methylimidazolium
Propionate Ionic Liquids {[C<sub><i>n</i></sub>mim][Pro](<i>n</i> = 3, 4, 5, 6)} from 288.15 K to 328.15 K
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Abstract
Four propionic acid ionic liquids
(PrAILs) [C<sub><i>n</i></sub>mim][Pro] (<i>n</i> = 3, 4, 5, 6) (1-alkyl-3-methylimidazolium
propionate) were prepared by the neutralization method and characterized
by <sup>1</sup>H NMR spectroscopy and differential scanning calorimetry.
The values of their density, surface tension, and refractive index
were measured in the temperature range of 288.15–328.15 K.
Since the PrAILs can strongly form hydrogen bonds with water, the
small amount of water is difficult to remove from the PrAILs by conventional
methods. To eliminate the effect of the water impurity, the standard
addition method (SAM) was applied to these measurements. The dependence
of volumetric properties, surface properties, and molar refraction
on temperature were discussed. In terms of Kabo’s method the
molar enthalpies of vaporization, Δ<i>H</i><sup>V</sup>, for the PrAILs were estimated. As a new idea, it was put forward
that Δ<i>H</i><sup>V</sup> can be assumed to consist
of two parts: one is the contribution from induced dipole, Δ<i>H</i><sup>V</sup><sub><i>n</i></sub>, and the other
is the contribution from the permanent dipole of ion pair in ILs,
Δ<i>H</i><sup>V</sup><sub>μ</sub>. The values
of Δ<i>H</i><sup>V</sup><sub><i>n</i></sub> can be estimated in terms of refractive index data so that the values
of Δ<i>H</i><sup>V</sup><sub>μ</sub> can be
obtained and Δ<i>H</i><sup>V</sup><sub>μ</sub> may be seen as the measure of polarity for ILs